This invention relates to fused tricyclic cyclohepta[1,2-5b]pyridine compounds and related compounds that are inhibitors of tyrosine kinases, in particular the receptor tyrosine kinase MET, and are useful in the treatment of cellular proliferative diseases, for example cancer, hyperplasias, restenosis, cardiac hypertrophy, immune disorders and inflammation.
Studies on signal transduction pathways have generated various promising molecular targets for therapeutic inhibition in cancer therapy. Receptor tyrosine kinases (RTK) represent an important class of such therapeutic targets. Recently, members of the MET proto-oncogene family, a subfamily of receptortyrosine kinases, have drawn special attention to the association between invasion and metastasis. The MET family, including MET (also referred to as c-Met) and RON receptors, can function as oncogenes like most tyrosine kinases. MET has been shown to be overexpressed and/or mutated in a variety of malignancies. A number of MET activating mutations, many of which are located in the tyrosine kinase domain, have been detected in various solid tumors and have been implicated in invasion and metastasis of tumor cells.
The c-Met proto-oncogene encodes the MET receptor tyrosine kinase. The MET receptor is a 190 kDa glycosylated dimeric complex composed of a 50 kDa alpha chain disulfide-linked to a 145 kDa beta chain. The alpha chain is found extracellularly while the beta chain contains extracellular, transmembrane and cytosolic domains. MET is synthesized as a precursor and is proteolytically cleaved to yield mature alpha and beta subunits. It displays structural similarities to semaphoring and plexins, a ligand-receptor family that is involved in cell-cell interaction.
The natural ligand for MET is hepatocyte growth factor (HGF), a disulfide linked heterodimeric member of the scatter factor family that is produced predominantly by mesenchymal cells and acts primarily on MET-expressing epithelial and endothelial cells in an endocrine and/or paraendocrine fashion. HGF has some homology to plasminogen.
It is known that stimulation of MET via hepatocyte growth factor (also known as scatter factor, HGF/SF) results in a plethora of biological and biochemical effects in the cell. Activation of c-Met signaling can lead to a wide array of cellular responses including proliferation, survival, angiogenesis, wound healing, tissue regeneration, scattering, motility, invasion and branching morphogenesis. HGF/MET signaling also plays a major role in the invasive growth that is found in most tissues, including cartilage, bone, blood vessels, and neurons.
Various c-Met mutations have been well described in multiple solid tumors and some hematologic malignancies. The prototypic c-Met mutation examples are seen in hereditary and sporadic human papillary renal carcinoma (Schmidt, L. et al., Nat. Tenet. 1997, 16, 68-73; Jeffers, M. et al., Proc. Nat. Acad. Sci. 1997, 94, 11445-11500). Other reported examples of c-Met mutations include ovarian cancer, childhood hepatocellular carcinoma, metastatic head and neck squamous cell carcinomas and gastric cancers. HGF/MET has been shown to inhibit anoikis, suspension-induced programmed cell death (apoptosis), in head and neck squamous cell carcinoma cells.
MET signaling is implicated in various cancers, especially renal. The nexus between MET and colorectal cancer has also been established. Analysis of c-Met expression during colorectal cancer progression showed that 50% of the carcinoma specimens analyzed expressed 5-50-fold higher levels of MET mRNA transcripts and protein versus the adjacent normal colonic mucosa. In addition, when compared to the primary tumor, 70% of colorectal cancer liver metastasis showed MET overexpression.
MET is also implicated in glioblastoma. High-grade malignant gliomas are the most common cancers of the central nervous system. Despite treatment with surgical resection, radiation therapy, and chemotherapy, the mean overall survival is <1.5 years, and few patients survive for >3 years. Human malignant gliomas frequently express both HGF and MET, which can establish an autocrine loop of biological significance. Glioma MET expression correlates with glioma grade, and an analysis of human tumor specimens showed that malignant gliomas have a 7-fold higher HGF content than low-grade gliomas. Multiple studies have demonstrated that human gliomas frequently co-express HGF and MET and that high levels of expression are associated with malignant progression. It was further shown that HGF-MET is able to activate Akt and protect glioma cell lines from apoptotic death, both in vitro and in vivo.
RON shares a similar structure, biochemical features, and biological properties with MET. Studies have shown RON overexpression in a significant fraction of breast carcinomas and colorectal adenocarcinomas, but not in normal breast epithelia or benign lesions. Cross-linking experiments have shown that RON and MET form a non-covalent complex on the cell surface and cooperate in intracellular signaling. RON and MET genes are significantly co-expressed in ovarian cancer cell motility and invasiveness. This suggests that co-expression of these two related receptors might confer a selective advantage to ovarian carcinoma cells during either tumor onset or progression.
A number of reviews on MET and its function as an oncogene have recently been published: Cancer and Metastasis Review 22:309-325 (2003); Nature Reviews/Molecular Cell Biology 4:915-925 (2003); Nature Reviews/Cancer 2:289-300 (2002).
Since dysregulation of the HGF/MET signaling has been implicated as a factor in tumorgenesis and disease progression in many tumors, different strategies for therapeutic inhibition of this important RTK molecule should be investigated. Specific small molecule inhibitors against HGF/MET signaling and against RON/MET signaling have important therapeutic value for the treatment of cancers in which Met activity contributes to the invasive/metastatic phenotype.